Minor modifications for skidding and nitro.

Still could be better, I'm not an artist ;p

data/gfx/nitro.xml

@@ -1,11 +1,11 @@
 <?xml version="1.0"?>
-<particles emitter="box" box_x="0.05" box_y="0.05" box_z="0.05">
+<particles emitter="box" box_x="0.05" box_y="0.05" box_z="0.60">
 
     <spreading angle="2"         />
     
     <velocity  x="0.0"
                y="0.0"
-               z="-0.03"         />
+               z="-0.003"         />
     
     <material  file="nitro-particle.png"  />
     
@@ -14,8 +14,8 @@
                max="1700"          />
     
     <!-- Minimal and maximal lifetime of a particle, in milliseconds. -->
-    <lifetime  min="10"
-               max="20"          />
+    <lifetime  min="15"
+               max="25"          />
     
     <!-- Size of the particles -->
     <size      min="0.1"

data/gfx/skid1.xml

@@ -16,11 +16,11 @@
                max="1500"          />
     
     <!-- Minimal and maximal lifetime of a particle, in milliseconds. -->
-    <lifetime  min="20"
+    <lifetime  min="15"
                max="25"          />
     
     <!-- Size of the particles -->
-    <size      min="0.45"
+    <size      min="0.35"
                max="0.45"         />
     
     <color     min="255 255 255"

data/gfx/skid2.xml

@@ -16,11 +16,11 @@
                max="1500"          />
     
     <!-- Minimal and maximal lifetime of a particle, in milliseconds. -->
-    <lifetime  min="20"
+    <lifetime  min="15"
                max="25"          />
     
     <!-- Size of the particles -->
-    <size      min="0.45"
+    <size      min="0.35"
                max="0.45"         />
     
     <color     min="255 255 255"